15 Must-Know JavaScript Array Methods

15 useful method to get the max out of JavaScript Arrays

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Arrays are wonderful and a very particular type in JavaScript. There are many useful built-in properties and methods that will help you resolve any task which involves arrays. Today we are going to discuss 15 array methods every developer should know.

  • some()
  • every()
  • reduce()
  • map()
  • flat()
  • filter()
  • forEach()
  • findIndex()
  • find()
  • sort()
  • concat()
  • fill()
  • includes()
  • reverse()
  • flatMap()

Notice the list is not enumerated as I don’t believe one method is more important than the other, each of them will resolve a different problem, and thus is important we are familiar with all.


some()

The some() tests whether at least one element in the array passes the test implemented by the callback function. The callback function will receive 3 arguments, the item, the index, and the full array. Additionally, is possible to assign a value for this when executing the callback by using the argument thisArg.

Definition:

arr.some(callback(element[, index[, array]])[, thisArg])

Examples:

const a = [1, 2, 3, 5, 8].some(item => item > 5)
const b = [1, 2, 3, 4, 5].some(item => item > 5)

console.log(a)
console.log(b)


---------
Output
---------
> true
> false

every()

The every() method is in a way similar to the some() method, but it tests whether all the elements in the array pass the test implemented by the callback function.

Definition:

arr.every(callback(element[, index[, array]])[, thisArg])

Examples:

const a = [10, 9, 8, 7, 6].every(item => item > 5)
const b = [7, 6, 5].every(item => item > 5)

console.log(a)
console.log(b)


---------
Output
---------
> true
> false

reduce()

The reduce() method executes a callback function once for each assigned value present in the array, taking 4 arguments:

  1. accumulator
  2. currentValue
  3. currentIndex
  4. array

The first time the callback is called, accumulator and currentValue can be either the initialValue if provided, and the first value in the array if not.

Definition:

arr.reduce(callback( accumulator, currentValue[, index[, array]] )[, initialValue])

How reduce() works

Let’s see with an example how reduce() works:

[0, 1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) => accumulator + currentValue)

If we go step by step and put in a table all the parameters plus the resulting value of the callback, we would get the following:

# accumulator currentValue currentIndex array return value
1 0 1 0 [0, 1, 2, 3, 4] 1
2 1 2 1 [0, 1, 2, 3, 4] 3
3 3 3 2 [0, 1, 2, 3, 4] 6
4 6 4 3 [0, 1, 2, 3, 4] 10

And the final result would be 10. In our particular case I did not provide an initial value, let’s try that next

const initialValue = 10
[0, 1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) => accumulator + currentValue, initialValue)

With this new scenario our table would like:

# accumulator currentValue currentIndex array return value
1 10 0 0 [0, 1, 2, 3, 4] 10
2 10 1 1 [0, 1, 2, 3, 4] 11
3 11 2 2 [0, 1, 2, 3, 4] 13
4 13 3 3 [0, 1, 2, 3, 4] 16
5 16 4 4 [0, 1, 2, 3, 4] 20

And the final resulting value is 20.

The reduce() function is great and it has several uses like sum all the values of an array or in an object array, counting for particular items in the array, grouping objects, merging arrays contained in array of objects, removing duplicates, etc.


map()

The map() method creates a new array populated with the results of the callback function for each element in the array. Similar to the other methods, the callback function will receive 3 arguments, currentValue, index, and array. As is with the case of reduce(), the callback is only invoked for indexes of the array which have assigned values (including undefined).

Definition:

arr.map(callback( currentValue[, index[, array]])[, thisArg])

Always be careful when using map(), remember that on each call will create a new array, if you don’t actually need the array and you are simply trying to iterate, use forEach() or for-of instead.

Examples:

const numbers = [1, 2, 3, 4, 5]
const doubled = numbers.map(value => value * 2)

console.log(doubled)

---------
Output
---------
> (5) [2, 4, 6, 8, 10]

As we mentioned map() will create a new array, so the following is a consequence of that:

const numbers = [1, 2, 3, 4, 5]
const numbers2 = numbers.map(value => value)
console.log('numbers', numbers)
console.log('numbers2', numbers2)
console.log(numbers === numbers2)

---------
Output
---------
numbers (5) [1, 2, 3, 4, 5]
numbers2 (5) [1, 2, 3, 4, 5]
false

Even though each array contains the exact same elements, they are not the same reference and thus the numbers === numbers2 resolves to false.


flat()

The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. By default, it will flatten 1 level.

Definition:

arr.flat([depth])

Examples:

const arr1 = [1, 2, [3, 4]]
console.log(arr1.flat())

const arr2 = [1, 2, [3, 4, [5, 6]]]
console.log(arr2.flat())

const arr3 = [1, 2, [3, 4, [5, 6]]]
console.log(arr3.flat(2))

const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]]
console.log(arr4.flat(Infinity))

---------
Output
---------
> (4) [1, 2, 3, 4]
> (5) [1, 2, 3, 4, Array(2)]
> (6) [1, 2, 3, 4, 5, 6]
> (10) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Note that if we want to flatten all levels recursively we can pass Infinity as the argument of the function.


filter()

Together with map() I think it’s one of my favorites. The filter() method creates a new array with all the elements that pass the test implemented by the callback function.

Definition:

arr.filter(callback(element[, index, [array]])[, thisArg])

Examples:

const array = [-3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

function isPrime(num) {
  for (let i = 2; num > i; i++) {
    if (num % i == 0) {
      return false
    }
  }
  return num > 1
}

console.log(array.filter(isPrime))

---------
Output
---------
> (6) [2, 3, 5, 7, 11, 13]

forEach()

The forEach() method executes a provided function once for each array element.

Definition:

arr.forEach(callback(currentValue [, index [, array]])[, thisArg])

Example:

const array = [1, 2, 3, 4, 5]
array.forEach((item) => console.log(item))

---------
Output
---------
1
2
3
4
5

There are 2 important considerations when using forEach()

  • There is no way to stop or break a forEach() loop other than throwing an exception.
  • forEach() expects a synchronous callback, it won’t wait for promises to be resolved.

Let’s see an example of the latter:

let ratings = [5, 4, 5]
let sum = 0

let sumFunction = async function (a, b)
{
  return a + b
}

ratings.forEach(async function(rating) {
  sum = await sumFunction(sum, rating)
})

console.log(sum)

---------
Output
---------
0

Even though we would have expected the variable sum to have accumulated all the values in the array and have a value of 14, the output was 0 as the forEach() statement ended without awaiting for the promises, and thus the console.log statement was executed before the variable sum was updated. So be very aware of this situation as it can lead to your code producing unexpected results.


findIndex()

The findIndex() method returns the index of the first element in the array that satisfies the provided callback function. Otherwise, it returns -1, indicating that no element passed the test. Unlike with other methods, findIndex() will execute the callback function even for indexes with unassigned values.

Definition:

arr.findIndex(callback( element[, index[, array]] )[, thisArg])

Example:

function isPrime(num) {
  for (let i = 2; num > i; i++) {
    if (num % i == 0) {
      return false
    }
  }
  return num > 1
}

console.log([4, 6, 8, 9, 12].findIndex(isPrime))
console.log([4, 6, 7, 9, 12].findIndex(isPrime))

---------
Output
---------
-1
2

find()

The find() method is similar to the findIndex() method, however, it returns the value of the first element which satisfies the provided callback function as supposed to its index. If no element satisfies the callback then undefined is returned.

Definition:

arr.find(callback(element[, index[, array]])[, thisArg])

Example:

const inventory = [
  {name: 'apples', quantity: 2},
  {name: 'bananas', quantity: 0},
  {name: 'cherries', quantity: 5}
]

const result = inventory.find( ({ name }) => name === 'cherries' )

console.log(result)


---------
Output
---------
> {name: "cherries", quantity: 5}

sort()

The sort() function is very common and simply allows us to sort the elements of an array in place and returning the sorting array. The default sort order is ascending. The complexity and performance of this method cannot be guaranteed as it depends on the implementation.

Definition:

arr.sort([compareFunction])

Examples:

const numbers = [4, 2, 5, 1, 3]
numbers.sort((a, b) => a - b)
console.log(numbers)

---------
Output
---------
> (5) [1, 2, 3, 4, 5]

Always remember that the sorting happens in place, so:

const numbers = [4, 2, 5, 1, 3]
const numbers2 = numbers.sort((a, b) => a - b)

console.log('numbers', numbers)
console.log('numbers2', numbers2)
console.log(numbers === numbers2)

---------
Output
---------
numbers > (5) [1, 2, 3, 4, 5]
numbers2 > (5) [1, 2, 3, 4, 5]
true

The sort function will modify the existing array, and return a reference to the same array, and thus the original array and the returned array will be the same.


concat()

The concat() method is used to merge two or more arrays into a new array.

Definition:

const new_array = old_array.concat([value1[, value2[, ...[, valueN]]]])

Examples:

const letters = ['a', 'b', 'c']
const numbers = [1, 2, 3]

console.log(letters.concat(numbers))

---------
Output
---------
> (6) ["a", "b", "c", 1, 2, 3]

fill()

The fill() method changes all the elements in an array to a static value, from a start index (default 0) to an end index (default array.length). The updates will happen in place and will return a reference to the same array.

Definition:

arr.fill(value[, start[, end]])

Examples:

const original = [1, 2, 3]
const result = original.fill(4)
console.log('original', original)
console.log('result', result)
console.log(original === result)

---------
Output
---------
original > (3) [4, 4, 4]
result > (3) [4, 4, 4]
true

includes()

The includes() method determines whether an array includes a certain value among its entries, returning true or false. Note that the method includes() is case-sensitive when comparing strings and characters.

Definition:

arr.includes(valueToFind[, fromIndex])

Examples:

console.log([1, 2, 3].includes(2))
console.log([1, 2, 3].includes(4))

---------
Output
---------
true
false

reverse()

The reverse() method reverses an array in place. By reversing we mean that the function will transpose the elements of the array, the first element will become the last, and the last the first element. This operation will mutate the original array and return a reference to the same.

Definition:

a.reverse()

Examples:

console.log([1, 2, 3].reverse())

---------
Output
---------
> (3) [3, 2, 1]

flatMap()

The flatMap() method applies a function to each element of the array and then flatten the result into an array. It combines flat() and map() in one function.

Definition:

arr.flatMap(callback(currentValue[, index[, array]])[, thisArg])

Example:

const array = [[1], [2], [3], [4], [5]]

const a = array.flatMap(arr => arr * 10)


// With .flat() and .map()
const b = array.flat().map(arr => arr * 10)

console.log('flatMap', a)
console.log('flat&map', b)

---------
Output
---------
flatMap (5) [10, 20, 30, 40, 50]
flat&map (5) [10, 20, 30, 40, 50]

Summary

JavaScript arrays come with some great methods that can simplify our development efforts. Knowing them can save us some time and in some cases even boost the performance of our code. I hope that today you learned some new array methods, or refresh on old concepts which you can use for your next project.

Please let me know on the comments if you come up with great functions that can be simplified using the methods above, and don’t forget to like the article if you enjoyed it, it helps me a lot when deciding which articles to write next.

Thanks for reading!

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